[Format] Fix detection of languages when reading from stdin (#79051)

The code cleanup in #74794 accidentally broke detection of languages by
reading file content from stdin, e.g. via `clang-format -dump-config - <
/path/to/filename`.

This PR adds unit and integration tests to reproduce the issue and adds
a fix.

Fixes: #79023

clang/test/Format/dump-config-objc-stdin.m

@@ -0,0 +1,5 @@
+// RUN: clang-format -dump-config - < %s | FileCheck %s
+
+// CHECK: Language: ObjC
+@interface Foo
+@end

clang/tools/clang-format/ClangFormat.cpp

@@ -547,18 +547,20 @@ static void PrintVersion(raw_ostream &OS) {
 // Dump the configuration.
 static int dumpConfig(bool IsSTDIN) {
   std::unique_ptr<llvm::MemoryBuffer> Code;
-  // We can't read the code to detect the language if there's no file name.
-  if (!IsSTDIN) {
-    // Read in the code in case the filename alone isn't enough to detect the
-    // language.
-    ErrorOr<std::unique_ptr<MemoryBuffer>> CodeOrErr =
-        MemoryBuffer::getFileOrSTDIN(FileNames[0]);
-    if (std::error_code EC = CodeOrErr.getError()) {
-      llvm::errs() << EC.message() << "\n";
-      return 1;
-    }
-    Code = std::move(CodeOrErr.get());
+
+  // `FileNames` must have at least "-" in it even if no file was specified.
+  assert(!FileNames.empty());
+
+  // Read in the code in case the filename alone isn't enough to detect the
+  // language.
+  ErrorOr<std::unique_ptr<MemoryBuffer>> CodeOrErr =
+      MemoryBuffer::getFileOrSTDIN(FileNames[0]);
+  if (std::error_code EC = CodeOrErr.getError()) {
+    llvm::errs() << EC.message() << "\n";
+    return 1;
   }
+  Code = std::move(CodeOrErr.get());
+
   llvm::Expected<clang::format::FormatStyle> FormatStyle =
       clang::format::getStyle(Style, IsSTDIN ? AssumeFileName : FileNames[0],
                               FallbackStyle, Code ? Code->getBuffer() : "");

clang/unittests/Format/FormatTestObjC.cpp

@@ -31,6 +31,14 @@ protected:
   _verifyIncompleteFormat(__FILE__, __LINE__, __VA_ARGS__)
 #define verifyFormat(...) _verifyFormat(__FILE__, __LINE__, __VA_ARGS__)
 
+TEST(FormatTestObjCStyle, DetectsObjCInStdin) {
+  auto Style = getStyle("LLVM", "<stdin>", "none",
+                        "@interface\n"
+                        "- (id)init;");
+  ASSERT_TRUE((bool)Style);
+  EXPECT_EQ(FormatStyle::LK_ObjC, Style->Language);
+}
+
 TEST(FormatTestObjCStyle, DetectsObjCInHeaders) {
   auto Style = getStyle("LLVM", "a.h", "none",
                         "@interface\n"